Glass Filling and Fracture Filling Explained

What is fracture filling in gemstones?

Fracture filling is a clarity treatment. A gem with cracks that reach the surface (gemmologists call them fractures or fissures) is soaked in a substance that flows into those cracks and then sets or stays put. The filler is chosen to have a refractive index (RI, a measure of how strongly a material bends light) close to the gem's own. When the two match, light passes straight through the crack instead of bouncing off it, so a white, glassy break becomes faint or vanishes.

Filling does not heal or close the crack. The fracture is still there, and the stone is just as weak along it as before; it is simply harder to see. That is the key difference from heat treatment, which changes colour and can partly heal fine fractures, and from dyeing, which adds colour rather than hiding cracks.

Filling turns up across many coloured gemstones and in some diamonds. The filler might be oil, wax, a synthetic resin or glass. Glass filling is the heavy-duty version: molten glass, often rich in lead, fills cracks and cavities that can run right through the stone. Every technical term used here is also defined in our jewellery glossary.

What is glass filling, and how is a glass-filled ruby made?

Lead-glass filling became widespread in ruby in the early to mid 2000s. The starting material is low-grade, heavily fractured corundum (the mineral that makes ruby and sapphire) that would otherwise have little use as a gem. It is cleaned in acid to open the fractures, then heated with a powdered glass containing lead oxide, sometimes with bismuth. The glass melts, is drawn into the fractures and cavities, and hardens as the stone cools.

Lead raises the RI of glass to about 1.7 or higher, close to ruby's 1.76–1.77, so the filled cracks nearly vanish. From arm's length the result looks like a transparent red ruby. In heavily filled stones, though, the glass can make up a large share of the volume and hold the stone together. For that reason, labs including GIA describe heavily treated material as a composite ruby or a manufactured product rather than simply a treated ruby.

• Lead-glass filled ruby: by far the most common glass-filled gem, often seen in low-priced rings, bracelets and earrings.
• Glass-filled sapphire: less common. Some blue material is filled with cobalt-coloured glass, which adds colour as well as hiding cracks.
• Flux-healed ruby and sapphire: a different, older process. Heating with a borax-type flux partly melts and re-crystallises the fracture walls, leaving small glassy residues. Reports call this heat treatment with residue, and it is far more stable than lead glass.
• Glass-filled diamond: a specialist high-RI glass in surface-reaching feathers, covered in the next section.

Most Australian sapphire, from central Queensland and the New England region of New South Wales, is heat-treated to lighten or brighten its colour, and glass filling is rarely seen in it. Lead-glass ruby is mostly made from low-grade rough from several African and Asian deposits; our guide to where rubies and sapphires are mined covers the main localities. Glass inside a filled gem is also a different thing from glass made to imitate a diamond, which is all glass and no gem.

Oil, resin, glass and wax: how do the fillers compare?

The closer the RI match, the better the cracks disappear, but no filler is as hard or as stable as the gem around it. Glass fillers sit around 5 to 6 on the Mohs scale (the 1–10 scale of scratch resistance), against 9 for ruby and 10 for diamond. Oils and resins are softer still, and they are organic, so they can age.

In emerald, oiling has been practised for centuries and is accepted across the trade, provided it is disclosed. Modern treaters often use resins instead because they last longer. Labs grade the amount of filling rather than just yes or no, usually on a scale such as none, minor, moderate and significant.

Diamond fracture filling dates from the 1980s. It is not the same as laser drilling, where a fine channel is burnt in to reach a dark inclusion so it can be bleached or dissolved. GIA does not issue grading reports for fracture-filled diamonds; our guide to laser drilling and fracture-filled diamonds explains why, and what to ask.

Which gemstones are commonly fracture filled?

Filling is used where a gem forms with many surface-reaching cracks, or where the rough is cheap enough that treatment is the only way to sell it. Here is how often you will meet it.

Some stones are impregnated rather than fracture filled: the resin soaks into a porous body instead of individual cracks. That describes most stabilised turquoise, the 'B jade' explained in our jadeite vs nephrite guide and the wax finish on much lapis lazuli in jewellery. Impregnation improves durability and polish as well as looks, but it still has to be disclosed.

Other gems are rarely treated this way. Iolite, the violet-blue cordierite, is usually sold untreated, as is Oregon sunstone with its copper schiller. Ametrine, the purple-and-gold quartz, is normally natural in clarity, although lab-created ametrine exists and must be sold as lab-created. Included pink beryl is occasionally filled; our morganite engagement ring pros and cons cover its usual heat treatment and what to ask.

How can you tell if a gemstone has fracture filling?

Some signs show up with a 10× loupe (a jeweller's magnifier) and a single light. Others need a lab. The usual clues:

• Flash effect: as you tilt the stone, a filled crack throws a flash of colour that moves and changes. In diamond it is typically orange-yellow against a dark background and blue-green against a light one. Glass-filled ruby often shows a bluish flash.
• Gas bubbles: round or flattened bubbles trapped in the filler, often in clusters, are typical of glass. Natural fluid inclusions are usually irregular in shape.
• Lustre difference: where a fracture reaches the surface, the glass or resin can look slightly duller than the gem around it in reflected light.
• Web-like cracks: a net of fine, partly hidden fractures across the whole stone is a strong hint of heavy glass filling in ruby.
• Dried filler in emerald: oil that has dried out leaves fractures looking white, yellowish or frosted.

A gem lab confirms filling with a microscope using darkfield illumination (light from the sides on a dark background), plus infrared spectroscopy (FTIR), Raman and X-ray fluorescence, which can identify an oil or resin or detect the lead in glass. Our overview of loupes, microscopes and lab instruments explains each one.

Is a gem with glass filling or fracture filling durable enough for a ring?

Durability comes down to hardness (resistance to scratching) and toughness (resistance to chipping and breaking). Filling does not change either property of the gem itself, but the filler is softer and more sensitive than the stone, and the hidden fractures remain weak points.

Feldspars such as moonstone and its blue sheen and labradorite's play of colour already have cleavage (a natural plane along which a crystal splits), so any filler in them is cosmetic only. Kunzite, the pink spodumene, has strong cleavage too and can fade in sunlight, so it suits pendants and earrings more than everyday rings.

What damages fillers is mostly what happens at the bench and the sink. A jeweller's torch during resizing or re-tipping, the acid 'pickle' used after soldering, and even acidic foods such as lemon juice can etch lead glass, turning the filled cracks cloudy and visible again. Ultrasonic and steam cleaners can strip oil from an emerald. A filled stone usually needs to come out of its setting before any repair involving heat.

Always tell the jeweller about any known filling before a resize, re-tip or polish, and clean filled gems with lukewarm water, mild soap and a soft brush only.

Is a fracture-filled or glass-filled gemstone worth buying?

Filling lowers value sharply because it hides, rather than removes, the cracks. As a rough guide, a lead-glass filled ruby of about 1 carat may sell for around AUD 20–200, while a natural, heated ruby of good colour might be roughly AUD 1,000–5,000 per carat, and fine unheated rubies far more. Emerald works differently: minor oiling is normal and expected, and prices step down as the filling rises from minor to significant.

A glass-filled ruby can make sense as inexpensive fashion jewellery, clearly disclosed and fairly priced. It is a poor fit for an engagement ring or an heirloom you expect to resize and repair over decades, because each visit to the bench is a risk.

If you want clean red or blue without the fragility, lab-created ruby and sapphire are another route. They are real corundum grown in a laboratory, usually very clean, need no filling and cost much less than natural stones; they must always be sold as lab-created or synthetic. For a colourless option, lab-created moissanite is harder than any filler and is not fracture filled. Natural gems keep the appeal of something formed in the earth; treated, natural and lab-created each have their place when they are described accurately.

What does a seller have to tell you about filled gems in Australia?

The Australian Consumer Law prohibits misleading or deceptive conduct, and leaving out a treatment that materially affects a gem's value or care can be misleading. International trade guidelines, such as those published by CIBJO (the world jewellery confederation), also require treatments to be disclosed. In practice, ask for the treatment to be written on the invoice or valuation, not just mentioned.

On a lab report, look for wording like this:

• No indications of clarity enhancement: no filler was detected.
• Minor / moderate / significant: the amount of oil or resin in an emerald.
• Lead-glass filled or clarity enhanced with glass: a glass-filled ruby or sapphire.
• Composite ruby or manufactured product: heavily glass-filled material where the glass forms a large part of the stone.
• Heated, with residue in fissures: flux healing, not lead-glass filling.
• Clarity enhanced (fracture filled): a filled diamond, usually from a lab other than GIA.

Terms differ slightly between labs, so if the wording is unclear, ask the seller or the lab to explain it before you pay.

How to check a gemstone for filling before you buy: step by step

1. Ask in writing. Ask whether the stone is treated and, if filled, with what: oil, resin or glass. Get the answer on the invoice.
2. Look with a loupe and one light. Tilt the stone slowly and watch for a moving flash of blue, green or orange along cracks.
3. Check for bubbles and dull patches. Round bubbles or areas of lower lustre on the surface point to glass or resin.
4. Ask for a lab report. For any significant ruby, sapphire, emerald or diamond, a report that states clarity enhancement, or its absence, settles the question.
5. Match the price to the treatment. A filled stone priced like an untreated one is a warning sign.
6. Plan for care and repairs. Note the treatment with your valuation, avoid ultrasonic and steam cleaning, and tell your jeweller before any work involving heat.